The present invention relates to a method and apparatus for making a multiple part tubular member such as a split bearing and, in particular, the crankshaft bearing of a connecting rod.
Connecting rods for internal combustion engines include a multiple part crank bearing having a separable semi-cylindrical cap which is bolted or otherwise releasably interconnected to a mating saddle integrally cast with the rod shank. A very simple method of manufacturing connecting rods with the separable cap is to cast the connecting rod in an integral blank having a head portion with the desired circular bearing opening. The head portion is weakened along a cracking or breaking line and pressure is applied to the head to cause separation by fracturing or cracking along the weakened plane.
In one cracking method the connecting rod head is weakened by removal of metal in cracking plane and with a radial, centrally located lubrication opening. In another prior art method, a plurality of cracking holes are located extending laterally through the connecting flange portion in parallel alignment with the cracking or separating plane through the crank head. The connecting rod is clamped within a suitable fixture with the cap portion projecting outwardly from a supporting edge surface aligned with the break plane. A force is applied to the upper surface of the cap portion thereby forcing the cap downwardly onto the supporting edge and resulting in a fracturing along the cracking plane.
Although such system has been proposed and provides many advantageous results, particularly from the standpoint of the economical production of engine connecting rods, a cracking system repeatably producing cracked rods which can be used in a commercial engine has not been obtainable as a mass production line process. Thus, generally employing the suggested cracking procedures resulted in an approximate 25 percent waste factor as a result of crack-generated faults. Thus, secondary cracks which extend from the split surface may be generated. Such secondary cracks result in failure in use and such units are waste. Further, chips may be generated on the thrust face of the cracked surfaces which would tend to lead to mismatch in the assembly with some out of roundness and the like, all of which would contribute to a relatively low useful life. Further loss or waste results from the creation of unacceptable residual stress within the cap structure as a result of the cracking in some of the units. Thus, although the method of forming a connecting rod cap by cracking has many advantages from the standpoint of economic and production, such advantages have been significantly reduced by the difficulty in maintaining adequate quality control.